The present invention is concerned with filters and more particularly filters of the type including a filter media layer disposed within a tank of unfiltered liquid and in which the tank liquid is caused to pass through filter media layer and collected for return.
U.S. Pat. No. 4,407,720 also discloses such a filter which is adapted to filter liquid disposed in a settling tank such as is used for filtration of cutting fluids used in industrial production of machined parts.
The filter disclosed in that arrangement takes the form of a drum member having a filter media layer disposed over a series of axial passages with the drum member rotatably mounted on a horizontally disposed central tube. The interior of the tube is arranged to receive filtration flow through the filter media layer into the axial passages for return to the system. In order to establish a connection with an external pump, the central tube is clamped against an aperture in the sidewall of the tank with the aperture in turn in fluid communication with suitable fittings for connection with the circulation pump.
In this design, the drum is caused to be rotated so as to enable a backwashing arrangement to be employed which greatly reduces the need for removal of the drum for for cleaning purposes. In this arrangement, backwashing flow is directed successively through the axial passages by means of a stationary backwash flow porting member having an opening positioned to successively be in registry with open ends of the axial flow passages as the drum is rotated.
This arrangement has been highly successful in reducing the need for removal and cleaning of the drum to reduce the need for a separate storage tank since it does not shut down filtration through those axial passages which are not subjected to the backwashing flow during the rotation of the drum member. This general arrangement thus allows the elimination of separate storage tanks to provide a flow of filtered liquid during backwashing operation and also reduces the need for removal and cleaning of the drum itself.
The drum configuration produces a relatively great surface filtration area for the unit flow space required. However, the horizontal axis arrangement of the drum and the need for making a fluid connection to the tube adds to the complexity of the arrangement. Also, the drive for the drum entails use of a chain drive extending downwardly into the interior of the tank which makes the drive components relatively inaccessible without removal of the drum from the tank.
The horizontal configuration of the drum also precludes filtering flow to the interior of the drum since the accumulated solids cannot be effectively removed, and the heavier particles cannot settle out in the tank. This limits the area available for filtering flow. Similarly, during the backwash operation, the contaminants would be trapped within the interior of the drum and could not be effectively removed from the interior space within the drum member.
The backwashing process itself, in most filtering arrangements, does not effectively remove the "fines" or smaller sized contaminants in a tank type filter unit, that is, a backwashing is often combined with scraping to remove the solid materials accumulating at the surface of the filter media.
In a tank type filter, the heavier particles sink to the bottom of the settling tank and are removed by a suitable means such as a drag conveyor within the interior. However, the smaller, lighter particles have a tendency to remain in suspension within the liquid and thus are drawn immediately again to the surface of the filter media layer upon resumption of normal filtering flow.
In industrial tank type filter installation, problems have been encountered due to fluctuations in the liquid level in the tank. If the filtering element becomes uncovered, air is drawn into the pumping system, causing interruptions in the flow of filtered liquid. Even if the liquid approaches the filter element, vortices may form, which also can allow entry of air into the pumping system with similar results. The tanks thus had to be sized to contain a sufficient volume of liquid to insure a minimum level throughout the cycle of the apparatus. In many instances, such tanks are positioned in excavations, such that the increased volume tanks added considerably to the expense of the filtering apparatus.